Omeka 2.6’s release notes mention that you need to update any plugin that uses TinyMCE:

Note: internal code and filenames have changed in this version of TinyMCE, so all plugins using HTML editors must be updated. Updated versions of ExhibitBuilder and SimplePages are bundled with the release, but others must be updated, including Commenting, Contribution, Posters, SimpleContactForm, and UserProfiles.